Current Research | Renewable Energy

Flow Control for VATT by Fixed and Oscillating Flap

By Wendi Liu, Dr Qing Xiao

This research is aimed to explore the potential to improve Vertical Axis Tidal Turbine (VATT) energy harnessing efficiency by using modified blades with fixed and oscillating flap. The fixed flap concept is borrowed from its application in aerodynamics area for reaching a high lift force at low flying speed. Oscillating flap is motivated by our relevant biomimetic studies on the flapping wing propulsion or energy extraction. Present investigation is concentrated on a VATT with NACA 0018 profile blade as its baseline turbine. Numerical simulations are carried out by solving incompressible Unsteady Navier-Stokes equations at turbulence flow condition. Computed results show that under certain optimal flap geometry and flow conditions, turbine power coefficient reaches 28% enhancement as compared to the conventional blade turbine. Detailed analysis on the flow structure demonstrates that this is related to the effective flow separation suppression and vortex control by applying a fixed and oscillating flap.

In this study, a new concept via the use of a fixed and oscillating flap to moderate the traditional H-shaped VATT blade is proposed. By a systematic numerical modeling investigation, the potential benefit to improve the VATT energy extraction efficiency is explored. Simulation results indicate that the fixed and oscillating flap techniques indeed enhance the VATT performance significantly, especially at large tip speed ratio, where the boundary layer separation dominates the flow. In addition, it is found that an oscillating flap presents a better performance than that of a fixed flap. The parametric study performed provides the optimal geometry parameters under the real operating conditions. For a fixed flap blade, these include a slot angle of 60°, slot width of 0.03c, slot location at 0.7c. With an oscillating flap, additional parameters include the oscillating amplitude of 15° and the revolution ratio of 1.5. With the above parameters, the peak power coefficient (Cp) rises about 28% relative to the full blade turbine and the thread shed of tip speed ratio at which the Cp exhibits decaying trend increases about 43% as well. Examination on the flow details around the rotating blade indicates that the dynamic stall associated with turbine blade at large AOA is effectively delayed with the use of fixed blade. With an oscillating flap, it provides one additional influence on reducing the blade wake vortex interaction. Both of the above two factors are beneficial in order to increase the blade lift coefficient, momentum coefficient and thus the overall efficiency of turbine.

Please refer to:

Qing Xiao, Wendi Liu and Atilla Incecik. "Flow control for VATT by fixed and oscillating flap." Renewable Energy 51 (2013): pp 141-152.

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